1. Field of the Invention
The invention pertains to traffic alert and collision avoidance systems, and more particularly, to such systems that cooperatively function with Air Traffic Control Radar Beacon Systems which provide intruder aircraft radar beacon surveillance.
2. Description of the Prior Art
A Traffic Alert and Collision Avoidance System (TCAS) installed on an aircraft cooperates with Air Traffic Control (ATC) by utilizing the ATC Radar Beacon Systems (ATCRBS) transponders installed in most aircraft worldwide and the new Mode-S transponders to provide locations of other aircraft in the immediate vicinity of the TCAS equipped aircraft. TCAS provides surveillance within the ATC System by transmitting interrogating signals to the ATCRBS equipped aircraft and measuring the relative range, altitude, and bearing from the responses of the ATCRBS transponder. The measured data and the rate of change of this data are utilized by TCAS to provide a prediction of aircraft penetration into a predetermined TCAS protection volume within a subsequent approximate thirty seconds of flight time. When an aircraft penetration is predicted within this time, an advisory is given to the pilot to climb to a new altitude, maintain his present altitude, or descend from his present altitude.
A problem with this system is that ATCRBS equipped aircraft respond to a non-unique interrogation. Thus, all ATCRBS transponders answer to the same interrogation when detected. Mode-S transponders respond to a unique address and may therefore be individually interrogated. To provide adequate surveillance of ATCRBS equipped aircraft, a procedure has been developed that segments the air space in the manner that permits only a limited number of ATCRBS equipped aircraft to respond at one time. This procedure, known as Whisper-Shout interrogates aircraft with increasingly stronger signals so that aircraft closer in range to the interrogating aircraft reply before aircraft located at greater ranges. ATCRBS transponders reply after two interrogation pulses, spaced twenty-one microseconds apart, have been detected. To prevent the closer proximity aircraft from replying to the stronger signals, a pulse having an amplitude lower than the interrogation pulse amplitudes is transmitted two microseconds before the first interrogation pulse. Initially, the first pulse, the suppression pulse, will not be detected and the transponder will reply after the reception of the two interrogation pulses. These transponders are designed to suppress a response when two pulses two microseconds apart are detected. Consequently, as the signal level of the interrogation is increased by the interrogating TCAS for detection by transponders at the longer ranges, the suppression pulse and the interrogation pulses will be detected at the closer ranges and the responses from the ATCRBS equipped aircraft at these closer ranges will be suppressed.
TCAS signals are transmitted by a phased array antenna which provides four sequential beam positions for performing 360.degree. surveillance. Each beam, however, contains sidelobes in which a close-in aircraft may receive the suppression pulse pair at a reduced amplitude from that of the main beam for which the first pulse is not detected, thus permitting the closer proximity aircraft outside the interrogation beam to respond to interrogation signals intended for aircraft at greater ranges inside the interrogation beam. To prevent such an occurrence, a sidelobe suppression pulse P2 is simultaneously transmitted with the suppression pulse pair on a cardioid beam having a notch at the peak of the main beam that is formed by coupling the P2 modulated RF signal to the antenna elements with appropriate phasing. The amplitude of the P2 pulse is such that it is detected only by the ATCRBS transponder on aircraft at closer ranges outside the interrogation beam, and when detected, suppresses the response from the ATCRBS transponder. The appropriate phasing of the antenna elements is established by applying P2 modulated RF signals with proper phase to three of the four input terminals of the beam forming network.
Current TCAS designs locate the active high power RF switches which direct the pulsed RF signals to the terminals of the antenna feed network in the TCAS computer unit located in an electronic's bay of the aircraft, thereby requiring long coaxial lines to couple the TCAS switches to the beam forming network of the antenna. Long coaxial lines do not provide stable phase matching characteristics. Systems of the prior art utilize calibration techniques to phase match the cables and firmly secure the cables to minimize phase variations due to cable vibrations. The present invention accomplishes sidelobe suppression and main beam narrowing with surveillance beams established by the beam forming network, thereby eliminating the requirement of phase matching long coaxial cables between the TCAS computer unit and the antenna.